Conventional resin molded products developed for reducing in the generation of a weld line is described, for example, in JP-B-4-27932. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
According to the above reference, a weld line 3 consisting only of a resin containing no metal particles is formed at the junction of resin flows as shown in FIG. 3, because each resin flow, during flowing, has a layer containing no metal particles at its front end.
There is in the above reference description that the weld line 3 is not conspicuous when the average distance (D) between the metal particles and the weld width (H) satisfy the relationship D.gtoreq.H and when the difference in appearance between each layer containing metal particles 91 and the layer not containing the same is small.
Incidentally, the following is an equation for calculating the average distance (D) between metal particles on the assumption that the metal particles are spherical. EQU D=R(.sup.3 .sqroot.Z-1)
wherein
D: average distance between the metal particles PA1 R: length corresponding to the average diameter of the metal particles PA1 Z=.pi./(3.sqroot.2.times.V) PA1 V: ratio of the volume of all metal particles to the total volume of the thermoplastic resin and the metal particles. PA1 (1) a resin composition comprising a thermoplastic resin and a brightening agent, wherein PA1 (2) a resin composition comprising a thermoplastic resin and a brightening agent, wherein PA1 (3) a resin molded product formed by die-molding a resin composition comprising a thermoplastic resin and a brightening agent, wherein PA1 (4) a resin molded product formed by die-molding a resin composition comprising a thermoplastic resin and a brightening agent, wherein
However, use of the above equation is unreasonable.
This is because the equation is not of a general character, since the aluminum particles for use in plastics and paints are generally not spherical and it is unusual to use spherical aluminum particles. In general, since the aluminum particles for use in plastics and paints have a flat scaly shape, the above relationship concerning the average distance (D) between metal particles does not hold for the aluminum particles. Consequently, the relationship with the weld width (H) also does not apply.
In the reference cited above, there is a description that the weld line part has a layer containing no metal particles and consisting only of a resin.
In contrast, the present inventors found, through close examination with a microscope, etc., that a resin molded product obtained from 1 part by weight of an aluminum powder (average particle size: 14 .mu.m) which is a scaly brightening agent as an example of a brightening agent and 100 parts by weight of polypropylene has an orientation extraordinary region 12 in which the scaly brightening agent 2 is oriented at an angle of from 45 degrees to a right angle and the orientation angle thereof becomes more close to a right angle as the position thereof is more close to the weld part, as shown in FIG. 1.
This orientation extraordinary region 12 less reflects and more absorbs light than the surrounding parts, and hence looks black.
This phenomenon in which the scaly brightening agent thus orients is not limited to the weld part, but occurs also in parts where the thickness changes, as shown in FIGS. 4 and 5.
As disclosed in JP-B-6-4718, the metal particles which are present in a weld line are distributed at a slightly lower density than those which are present in ordinary parts and are oriented perpendicularly to the surface of the resin molded product. Due to this, the resin molded product, according to the reference, appears to have considerable unevenness of color density especially when the size of the metal particles is not larger than 10 .mu.m.
The present inventors, however, have ascertained the fact that when the addition amount of metal particles exceeds 1 part by weight, the orientation extraordinary region becomes narrower as the proportion of finer metal particles increases and, as a result, the extraordinary weld appearance becomes less conspicuous.